1. Field of the Invention
The present invention relates to a fluidized bed equipment and a process for drying or cooling of powder by use of the equipment, which relates especially to an equipment and a process which enables with a remarkably high heat efficiency fluidized bed drying or cooling of an extremely fine powder or an extremely low density powder heretofore hardly processed under steady operating conditions with an economically feasible areal velocity due to their tendency of being entrained by the fluidizing air flow.
2. Description of the Prior Art
For conventional fluidized bed drying (cooling) equipments employing solely air as the heat transfer medium, the heat transferred per unit area of the air dispersing floor plate (grid) is determined by the difference in inlet and outlet air temperatures as well as the volume of air (areal velocity.times.time).
In the operation of fluidized bed equipments, the areal velocity is usually settled at a value around the maximum (the value above which no fluidized bed of powder is formed due to flying out of powder) for enhancing the cost-performance based on a larger coefficient of heat capacity to bring about a decreased floor plate area and an decreased cost of the fluidized bed equipment. However, the features and design principles bring about the following problems on conventional fluidized bed drying (cooling) equipments.
a) The larger the air dispersing floor plate areal velocity, the more the contact of powder with air becomes insufficient, which tends to cause larger differences between temperature of powder being-heated (cooled) in-the fluidized bed-and temperature of the gas passing through the bed. Though this results a large coefficient of heat capacity for the equipment, it brings about a reduced heat efficiency due to a decrease in effective air temperature differences (differences between inlet and outlet air temperatures). A thick fluidized bed is contemplated to overcome a large temperature difference between the powder and air, however, a large amount of powder must be retained in the bed and tends to cause uneven fluidization due fluctuation in bed thickness.
b) When an equipment is operated with an allowable hottest air for the highest cost-performance, degradation and scorching of retained powder tend to occur.
c) The heat efficiency is low, and a low heat efficiency of as low as less than 20% is observed especially for a low temperature fluidized bed drying of a thermally unstable powder.
d) A long period of time is necessary after the start up until reaching to stationary operating conditions.
e) A large size equipment is required for processing a large mount of material, due to a low heat efficiency.
f) The cost-performance is determined based on the coefficient of heat capacity being around 2000-6000 Kcal/m.sup.3 h.degree.C. for practical equipments, and below 1000 Kcal/m.sup.3 h.degree.C. is considered to be impractical commercially. From this reason, for conventional fluidized bed drying (cooling) equipments, fine powder having a air dispersing floor plate maximum areal velocity of less than 20 cm/s are recognized as out of the subject. In the above, the coefficient of heat capacity means the product of a coefficient of heat transfer and an effective heat transfer area per unit volume of equipment; the coefficient of heat transfer means the quantity of heat transferred per unit heat transfer area per unit length of time per unit temperature difference; and the heat efficiency means the ratio of quantity of heat used effectively to the total quantity of heat supplied.
An agitating-rotating-fluidization equipment having a horizontal semi-cylindrical bottom wall with numerous perforations and rotary heating discs being set in the semi-cylindrical bottom for heating and agitation is proposed, in which powder is fluidized by hot air blowing through the perforations and agitated by the rotary heating discs. Since the powder remains in thin layer on the semi-cylindrical perforated bottom wall when rotation of the discs is stopped, the blow-by of air therefrom is inevitable, and so it is required to make the discs rotate forcefully to stabilize the fluidization. Further, regarding the performance, only around a half of the surface area of heating discs effectively contributes to the heat transfer.
In another type of equipment having a group of vertical pipes in the fluidized bed, it is forced to reduce the ratio of the projected area of pipes to the area of air dispersing floor plate to be around 10% because of prevention of the hindered fluidization. Owing to the structure, the group of pipes requires a header at the bottom, which tends to be an obstacle to the fluidization. For this type of equipment, for example, in order to have a total surface area of pipes of two times of the air dispersing floor plate area, the fluidizing bed of powder must have a thickness of at least 500 mm. Structurally, the equipment is being employed only for granular particulate materials allowable to adopt a high air dispersing floor plate (grid) areal velocity, and thus the heat transfer through contact with the group of pipes is regarded as supplementary to the heat transferred by air. Though the superiority of this equipment may be recognizable, it is not evaluated by usual users as superior than ordinary fluidized bed drying (cooling) equipments employing air only as the heat transfer medium because of difficulties in the operability, washability and maintenance.
A fine powder or an ultra fine powder having a small true specific gravity is entrained well by air flow and a quite low areal velocity of air is required for obtaining a stably fluidized bed of the powder, which made such powder regarded as unsuitable for being dried or cooled with conventional fluidized bed drying or cooling equipments due to a low capacity and an inferior cost-performance coming from a large scale of the equipment.